The obtainment of recombinant products of vaccinal interest in different expression systems, such as bacteria, fungi, yeasts, plants, insect cells and larvae, mammalian cells, etc., has been known for some time. Amongst these systems, plants offer numerous advantages as compared to other expression systems, since, in general, they represent an economical, safe and easy-to-obtain method of obtaining proteins of potential pharmaceutical interest, for example, recombinant subunit vaccines, without the need for costly fermentation systems.
However, in all systems, the main limitation in the obtainment of vaccines from recombinant subunits of the pathogen is the low immunogenicity of the recombinant products obtained; as a result, very high, repeated vaccine doses are usually required to equal the immune response obtained with the conventional immunogen (complete deactivated pathogen). This circumstance makes the production costs of recombinant subunit vaccines very high as compared to conventional vaccines and, therefore, many of them do not currently reach the market.
Different alternatives have been followed in order to improve the immunogenicity of recombinant subunit vaccines. One of them is based on the use of CTLA4- and L-selectin-type molecules to direct vaccinal antigens toward antigen-presenting cells in mice [Boyle J. S. et al., Enhanced responses to a DNA vaccine encoding a fusion antigen that is directed to sites of immune induction. Nature. 1998, Mar. 26; 392 (6674): 408-411]. However, these strategies have not proven to be equally effective in species other than the murine species, which makes it necessary to search for other alternatives based on the directioning of vaccinal antigens toward the cells in charge of antigen presentation in other species, particularly the human species.